(a) Field of the Invention
The present invention relates to an imaging optical system which can be zoomed and is to be attached to an eyepiece of fiberscopes or non-flexible endoscopes for focusing images on image pick-up devices of a monitor.
(b) Description of the Prior Art
Composition of an instrument used for observing interiors of coeloms, machines, etc. on a TV monitor is illustrated in FIG. 1 wherein the reference numeral 1 represents a fiberscope which accommodates an observation optical system consisting of an objective lens 2, an image guide 3, an eyepiece lens 4, etc., and an illumination optical system consisting of a light source 5, a light guide 6, etc., and is used for observing objects through the observation optical system while illuminating the objects with the illumination optical system. In order to observe an image formed by the observation optical system of the fiberscope, an adapter lens 7 is arranged after the eyepiece lens 4 so that an image is formed on an image pick-up device 8 by the adapter lens and projected onto a TV set for observation.
Since diameters of the image guides and magnifications of the eyepiece lenses are different depending on types of fiberscopes used, the optical system for focusing images obtained with a fiberscope onto an image pick-up device by using an adapter lens and projecting the images on a monitor provides images of different sizes on the TV monitor 10.
In case of the ordinary TV signals of NTSC type, it is adequate to observe images from a distance 5 to 6 times as long as the vertical side of the TV monitor. Accordingly, images are not clearly observable when images are too small on the monitor. When images are too large on the monitor, in contrast, the images formed on an image guide are projected only partially on the monitor.
For the reason described above, it has conventionally been necessary to prepare a multiple number of adapter lenses having fixed and different magnifications, and exchange the adapter lenses so as to obtain images of adequate sizes on the monitor, thereby posing a problem of inconvenience.
Further, brightness for photographing (F number) is determined by a aperture stop for the eyepiece lens 4 in the fiberscope 1. Therefore, the F number is enlarged and images are darkened when an adapter lens having a high magnification is used. Accordingly, it is impossible to observe an object located at a long distance in such a case. In order to observe an object located at a long distance, it is necessary to use an adapter lens having a low magnification for brightening its image while sacrificing image size and convenience.
In order to solve the problems described above, it is strongly demanded to use a zoom lens system as the adapter lens. However, since image pick-up devices are prepared in multiple types having different image sizes of 1/2 inch, 2/3 inch, etc., sizes of images projected on a TV monitor are different depending on image sizes of image pick-up devices used even when images obtained with the same fiber scope are formed at the same magnification on the image pick-up devices. Accordingly, variation range of image size projected on the monitor is not uniformalized due to the variation of image sizes of the image pick-up devices and images may not be observed at adequate sizes even when a zoom lens system is used as the adapter lens.
In order to allow to use image pick-up devices having various image sizes and to project images formed with fiber bundles having various diameters onto the monitor at an adequate size, it is necessary to use an adapter lens having a high zooming ratio. When a zoom lens system used as the adapter lens has a high zooming ratio, however, the zoom lens system has undesirably large total length.
In the recent years, most image pick-up devices are of two types having image sizes of 2/3 inch and 1/2 inch, and image pick-up devices of 1/2 inch are mainly used taking the compactness thereof into consideration. Since performance such as resolution and brightness of image pick-up devices is improved, image pick-up devices of 1/2 inch are satisfactory in performance thereof. In view of the fact that the imaging optical system is used in various ways, it is necessary to adopt an adapter lens compatible with image pick-up devices of any image sizes.
Furthermore, when CCD or the similar element is used as the image pick-up device, it is necessary to arrange an optical low pass filter consisting of a birefringent plate such as crystal before the CCD, and the adapter lens must have a long back focal length.